Plane type stripline filter and dual mode resonator

Abstract: A strip-line filter is provided with upper- and lower-stage resonators having the same electromagnetic characteristics. Each of the resonators has a one-wavelength square-shaped strip line and four open-end transmission lines connected to four coupling points A,C,B and D (or E,G,F and H) of each resonator which are spaced 90 degrees in electric length in that order. The square-shaped strip lines have a pair of parallel coupling lines closely placed in parallel to each other to electromagnetically couple the resonators. Therefore, the filter can be manufactured in a small size. A first microwave resonated in each resonator is electromagnetically influenced by two open-end transmission lines connected to two coupling points A and B (or E and F), and a second microwave resonated in each resonator is electromagnetically influenced by two open-end transmission lines connected to two coupling points C and D (or G and H). Therefore, resonance wavelengths of the microwaves can be longer than a line length of each square-shaped strip line. Also, the resonance wavelengths can be adjusted by trimming the transmission lines. Also, because all constitutional elements are made of strip lines, the filter can be made plane.

Description: The present invention relates generally to a strip-line filter utilized to filter microwaves in a communication apparatus or a measuring apparatus operated in frequency bands ranging from an ultra high frequency (UHF) band to a super high frequency (SHF) band, and more particularly to a strip-line filter in which a strip line is shortened and is made plane at low cost. Also, the present invention relates generally to a dual mode resonator utilized for an oscillator or a strip-line filter, and more particularly to a dual mode resonator in which two types microwaves are independently resonated. A strip-line resonating filter is manufactured by serially arranging a plurality of one-wavelength type of strip line ring resonators to reduce radiation loss of microwaves transmitting through a strip line of the resonating filter. However, there is a drawback in the strip-line resonating filter that the resonating filter cannot be downsized. Therefore, a dual mode strip-line filter in which microwaves in two orthogonal modes are resonated and filtered has been recently proposed. A conventional dual mode strip-line filter is described with reference to Figs. 1 and 2. Fig. 1 is a plan view of a conventional dual mode strip-line filter. Fig. 2A is a sectional view taken generally along the line II-II of Fig. 1. Fig. 2B is another sectional view taken generally along the line II-II of Fig. 1 according to a modification. The strip line filter of Figure 1 is described further in EP-0-573,985 A1. As shown in Fig. 1, a conventional dual mode strip-line filter 11 comprises an input terminal 12 excited by microwaves, a one-wavelength strip line ring resonator 13 in which the microwaves are resonated, an input coupling capacitor 14 connecting the input terminal 12 and a coupling point A of the ring resonator 13 to couple the input terminal 12 excited by the microwaves to the ring resonator 13 in capacitive coupling, an output terminal 15 which is excited by the microwaves resonated in the ring resonator 13, an output coupling capacitor 16 connecting the output terminal 15 and a coupling point B in the ring resonator 13 to couple the output terminal 15 to the ring resonator 13 in capacitive coupling, a chase-shifting circuit 17 coupled to a coupling point C and a coupling point D of the ring resonator 13, a first coupling capacitor 18 for coupling a connecting terminal 20 of the phase-shifting circuit 17 to the coupling point C in capacitive coupling, and a second coupling capacitor 19 for coupling another connecting terminal 21 of the phase-shifting circuit 17 to the coupling point D in capacitive coupling. The ring resonator 13 has a uniform line impedance and an electric length which is equivalent to a resonance wavelength λ o . In this specification, the electric length of a closed loop-shaped strip line such as the ring resonator 13 is expressed in an angular unit. For example, the electric length of the ring resonator 13 equivalent to the resonance wavelength λ o is called 360 degrees. The input and output coupling capacitors 14, 16 and first and second coupling capacitors 18, 18 are respectively formed of a plate capacitor. The coupling point B is spaced 90 degrees in the electric length (or a quarter-wave length of the microwaves) apart from the coupling point A. The coupling point C is spaced 180 degrees in the electric length (or a half-wave length of the microwaves) apart from the coupling point A. The coupling point D is spaced 180 degrees in the electric length apart from the coupling point B. The phase-shifting circuit 17 is made of one or more passive or active elements such as a capacitor, an inductor, a strip line, an amplifier, a combination unit of those elements, or the like. A phase of the microwaves transferred to the phase-shifting circuit 17 shifts by a multiple of a half-wave length of the microwaves to produce phase-shift microwaves. As shown in Fig. 2A, the ring resonator 13 comprises a strip conductive plate 22, a dielectric substrate 23 mounting the strip conductive plate 22, and a conductive substrate 24 mounting the dielectric substrate 23. That is, the ring resonator 13 is formed of a microstrip line. The wavelength of the microwaves depends on a relative dielectric constant ε r of the dielectric substrate 23 so that the electric length of the ring resonator 13 depends on the relative dielectric constant ε r . In a modification, the ring resonator 13 is formed of a balanced strip line shown in Fig. 2B. As shown in Fig. 2B, the ring resonator 13 comprises a strip conductive plate 22m, a dielectric substrate 23m surrounding the strip conductive plate 22m, and a pair of conductive substrates 24m sandwiching the dielectric substrate 23m. In the above configuration, when the input terminal 12 is excited by microwaves having various wavelengths around the resonance wavelength λ o , electric field is induced around the input coupling capacitor 14 so that the intensity of the electric field at the coupling point A of the ring resonator 13 is increased to a maximum value. Therefore, the input terminal 12 is coupled to the ring resonator 13 in the capacitive coupling, and the microwaves are transferred from the input terminal 12 to the coupling point A of the ring resonator 13. Thereafter, the microwaves are circulated in the ring resonator 13 in clockwise and counterclockwise directions. In this case, the microwaves having the resonance wavelength λ o are selectively resonated according to a first resonance mode. The intensity of the electric field induced by the microwaves resonated is minimized at the coupling point B spaced 90 degrees in the electric length apart from the coupling point A because the intensity of the electric field at the coupling point A is increased to the maximum value. Therefore, the microwaves are not directly transferred to the output terminal 15. Also, the intensity of the electric field is minimized at the coupling point D spaced 90 degrees in the electric length apart from the coupling point A so that the microwaves are not transferred from the coupling point D to the phase-shifting circuit 17. In contrast, because the coupling point C is spaced 180 degrees in the electric length apart from the coupling point A, the intensity of the electric field at the coupling point C is maximized, and the connecting terminal 20 is excited by the microwaves circulated in the ring resonator 13. Therefore, the microwaves are transferred from the coupling point C to the phase-shifting circuit 17 through the first coupling capacitor 18. In the phase-shifting circuit 17, the phase of the microwaves shifts to produce phase-shift microwaves. For example, the phase of the microwaves shifts by a half-wave length thereof. Thereafter, the connecting terminal 21 is excited by the phase-shift microwaves, and the phase-shift microwaves are transferred to the coupling point D through the second coupling capacitor 19. Therefore, the intensity of the electric field at the coupling point D is increased to the maximum value. Thereafter, the phase-shift microwaves are circulated in the ring resonator 13 in the clockwise and counterclockwise directions so that the phase-shift microwaves are resonated according to a second resonance mode. Thereafter, because the coupling point B is spaced 180 degrees in the electric length apart from the coupling point D, the intensity of the electric field is increased at the coupling point B. Therefore, electric field is induced around the output coupling capacitor 16, so that the output terminal 15 is coupled to the coupling point B in the capacitive coupling. Thereafter, the phase-shift microwaves are transferred from the coupling point B to the output terminal 15. In contrast, because the coupling points A, C are respectively spaced 90 degrees in the electric length apart from the coupling point D, the intensity of the electric field induced by the phase-shift microwaves is minimized at the coupling points A, C. Therefore, the phase-shift microwaves are transferred to neither the input terminal 12 nor the connecting terminal 20. Accordingly, the microwaves having the resonance wavelength λ o are selectively resonated in the ring resonator 13 and are transferred to the output terminal 15. Therefore, the conventional dual mode strip-line filter 11 functions as a resonator and filter. The microwaves transferred from the input terminal 12 are initially resonated in the ring resonator 13 according to the first resonance mode, and the phase-shift microwaves are again resonated in the ring resonator 13 according to the second resonance mode. Also, the phase of the phase-shift microwaves shifts by 90 degrees as compared with the microwaves. Therefore, two orthogonal modes formed of the first resonance mode and the second resonance mode independently coexist in the ring resonator 13. Therefore, the conventional dual mode strip-line filter 11 functions as a two-stage filter. However, passband characteristics of the filter 11 is determined by the electric length of the ring resonator 13, so that a microwave having a fixed wavelength such as λ o is only resonated. Therefore, because the electric length of the ring resonator 13 is unadjustable, there is a drawback that the adjustment of the resonance wavelength is difficult. Also, because it is required that the electric length of the strip line ring resonator 13 is equal to the one wavelength λ o of the resonance microwave and because the phase-shifting circuit 17 is formed of a concentrated constant element such as a coupling capacitor or a transmission line such as a strip line, there is another drawback that it is difficult to manufacture the filter 11 in a small-size and plane shape. Fig. 3 is a plan view of another conventional dual mode strip-line filter. As shown in Fig. 3, another conventional dual mode stripline filter 31 comprises two dual mode strip-line filters 11 arranged in series. An inter-stage coupling capacitor 32 is connected between the coupling point D of the filter 11 arranged at an upper stage and the coupling point A of the filter 11 arranged at a lower stage. The phase-shifting circuit 17 of the filter 11 arranged at the upper stage is composed of a coupling capacitor 33, and the phase-shifting circuit 17 of the filter 11 arranged at the lower stage is composed of a coupling capacitor 34. In the above configuration, when the input terminal 12 is excited by a signal (or a microwave) having a resonance wavelength λ o , the signal is resonated according to the first and second resonance modes in the same manner, and the signal is transferred to the coupling point A of the filter 11 arranged at the lower stage through the inter-stage coupling capacitor 32. Thereafter, the signal is again resonated according to the first and second resonance modes in the filter 11 arranged at the lower stage, and the signal is output from the coupling point D to the output terminal 15. In this case, the resonance wavelength λ o is determined according to an electric length of the ring resonator 13. Therefore, the conventional dual mode strip-line filter 31 functions as a four-stage filter in which the signal is resonated at four stages arranged in series. However, it is required that the electric length of the strip line ring resonator 13 is equal to the one wavelength λ o of a resonance microwave, and it is required to increase the number of filters 11 for the purpose of improving attenuation characteristics of the resonance microwave. Therefore, there is a drawback that a small sized filter cannot be manufactured. Also, the phase-shifting circuit 17 is formed of a concentrated constant element such as a coupling capacitor or a transmission line such as a strip line, there is another drawback that it is difficult to manufacture the filter 31 in a small-size and plane shape. A quarter-wavelength strip line resonator made of a balanced strip line or a micro-strip line has been broadly utilized in a high frequency band as an oscillator or a resonator utilized for a strip-line filter because the quarter-wavelength strip line resonator can be made in a small size. However, because ground processing in a high-frequency is performed for the quarter-wavelength strip line resonator, there are drawbacks that characteristics of a resonance frequency and a no-loaded Q factor (Q=ω o /2Δω. ω o denotes a resonance angular frequency and Δω denotes a full width at half maximum) vary. To solve the drawbacks, a dual mode resonator in which two types microwaves having two different frequencies are resonated or a microwave is resonated in two stages by utilizing two independent resonance modes occurring in a ring-shaped resonator not grounded in high-frequency has been proposed for the purpose of downsizing a resonator. The dual mode resonator is, for example, written in a technical Report MW92-115 (1992-12) of Microwave Research in the Institute of Electronics. Information and Communication Engineers. A conventional dual mode resonator is described with reference to Fig. 4. Fig. 4 is an oblique view of a conventional dual mode resonator. As shown in Fig. 4, a conventional dual mode resonator 41 comprises a rectangular-shaped strip line 42 for resonating two microwaves having two different frequencies f1 and f2, a lumped constant capacitor 43 connected to connecting points A, B of the rectangular-shaped strip line 42 for electromagnetically influencing the microwave having the frequency f1, a dielectric substrate 44 mounting the strip line 42, and a grounded conductive plate 45 mounting the dielectric substrate 44. Electric characteristics of the rectangular-shaped strip line 42 is the same as those of a ring-shaped strip line. The strip line 42 is made of a micro-strip line. However, it is applicable that the strip line 42 be made of a balanced strip line. In the above configuration, when a first input terminal (not shown) connected to the connecting point A is excited by a first signal (or a first microwave) having a frequency f1, an electric voltage at the connecting point A is increased to a maximum value. Therefore, the first signal is transferred from the first input terminal to the connecting point A of the strip line 42. Thereafter, the first signal is circulated in the strip line 42 in clockwise and counterclockwise directions in a first resonance mode. In this case, electric voltages at connecting points C and D spaced 90 degrees in the electric length (or a quarter-wave length of the first signal) apart from the connecting point A are respectively reduced to a minimum value, so that the first signal is not output from the connecting point C or D to a terminal (not shown) connected to the connecting point C or D. Also, an electric voltage at the connecting point B spaced 180 degrees in the electric length (or a half-wave length of the first signal) apart from the connecting point A is increased to the maximum value, so that the first signal is output from the connecting point B to a first output terminal (not shown) connected to the connecting point B. In contrast, when a second input terminal (not shown) connected to the connecting point C is excited by a second signal (or a second microwave) having a frequency f2, an electric voltage at the connecting point C is increased to a maximum value. Therefore, the second signal is transferred from the second input terminal to the connecting point C of the strip line 42. Thereafter, the second signal is circulated in the strip line 42 in clockwise and counterclockwise directions in a second resonance mode. In this case, electric voltages at the connecting points A and B spaced 90 degrees in the electric length apart from the connecting point C are respectively reduced to a minimum value, so that the second signal is not output from the connecting point A or B to the first input or output terminal connected to the connecting point A or B. Also, an electric voltage at the connecting point D spaced 180 degrees in the electric length apart from the connecting point C is increased to the maximum value, so that the second signal is output from the connecting point B to a second output terminal (not shown) connected to the connecting point D. Because any lumped constant capacitor connected to the connecting points C and D is not provided, the frequency f1 differs from the frequency f2. However, in cases where a capacitor having the same capacity as that of the capacitor 43 is provided to be connected between the connecting points C and D, the frequency f2 is equal to the frequency f1. Also, in cases where the capacitor 43 is removed, the frequency f1 is equal to the frequency f2. Therefore, the frequencies f1 and f2 resonated in the first and second resonance modes independent each other are the same. In other words, the conventional dual mode resonator 41 functions as a two-stage resonator in which two microwaves having the same frequency are resonated in two stages arranged in parallel. Accordingly, the resonator 41 comprising the strip line 42 and the capacitor 43 functions as a dual mode resonator in which two microwaves are resonated in two resonance modes independent each other. Because the resonator 41 is not grounded in high-frequency as a special feature of a dual mode resonator and because radiation loss of the microwave is lessened because of a closed-shape strip line as another special feature of the dual mode resonator, the resonator 41 can be manufactured in a small size without losing the special features of a one-wavelength ring-shaped dual mode resonator. However, it is required to accurately set a lumped capacity of the capacitor 43 for the purpose of obtaining a resonance frequency of a microwave at a good reproductivity. In actual manufacturing of the dual mode resonator 41, it is difficult to accurately set a lumped capacity of the capacitor 43. In cases where a frequency adjusting element is additionally provided for the dual mode resonator 41 to accurately set a lumped capacity of the capacitor 43, the number of constitutional parts of the dual mode resonator 41 is increased. Therefore, there are drawbacks that resonating functions of the resonator 41 are degraded and a manufacturing cost of the resonator 41 is increased. An aim of the present invention is to provide a strip-line filter in which attenuation characteristics of a microwave in the neighbourhood of a passband of the microwave is improved and a small sized filter is manufactured in a plane shape. According to the present invention there is provided a strip line filter for resonating and filtering a microwave signal, the filter comprising:- a series of one-wavelength loop-shaped strip line resonators respectively having a uniform line impedance for respectively resonating and filtering a microwave signal in a first resonance mode in which electric voltages at both a first coupling point and a second coupling point spaced 180 degrees in electric length apart from the first coupling point are maximized and respectively resonating and filtering the microwave signal in a second resonance mode in which electric voltages at both a third coupling point spaced 90 degrees in electric length apart from the first coupling point and a fourth point spaced 180 degrees in electric length apart from the third coupling point are maximized, each of the resonators having a first coupling line (L 2 ) between the first and third coupling points and a second coupling line (L 2 ) between the second and fourth coupling points; a microwave inputting element for inputting a microwave signal to the first coupling point of the resonator arranged in the first stage; and a microwave outputting element for outputting the microwave signal from the fourth coupling point of the resonator arranged in the final stage; the second coupling line of one resonator arranged in an N-th stage (N is an integral number) being electromagnetically coupled to the first parallel coupling line of another resonator arranged in an (N+1)-th stage to transfer the microwave signal from the resonator arranged in the N-th stage to the resonator arranged in the (N+1)-th stage; characterised by: four open-ended transmission lines connected to the first, second, third and fourth coupling points of each of the resonators for electromagnetically influencing the microwave signal resonated in each of the resonators, the open-ended transmission lines having the same electromagnetic characteristics; and an inter-stage coupling circuit for transferring the microwave signal resonating in the first resonance mode from the second coupling point of the resonator in the final stage to the third coupling point of the resonator arranged in the first stage so as to cause the microwave signal transferred by the inter-stage coupling circuit to resonate in the second resonance mode, the microwave signal resonating in the second resonance mode being output by the microwave outputting element. In the above configuration, in cases where a microwave resonated according to the first resonance mode (or the second resonance mode) is transferred to a one-wavelength loop-shaped strip line resonator arranged in an N-th stage, a second parallel coupling line of the one-wavelength loop-shaped strip line resonator arranged in the N-th stage is electromagnetically coupled to a first parallel coupling line of a one-wavelength loop-shaped strip line resonator arranged in an N+1)-th stage. Therefore, the microwave resonated is transferred by stages from a one-wavelength loop-shaped strip line resonator arranged in a first stage to another one-wavelength loop-shaped strip line resonator arranged in a final stage. When a microwave is transferred from the microwave inputting means to the first coupling point of the one-wavelength loop-shaped strip line resonator arranged in the first stage, the microwave is resonated and filtered according to the first resonance mode in each of the one-wavelength loop-shaped strip line resonators. In this case, the microwave is influenced by the open-end transmission lines connected to the first and second coupling points. Therefore, the microwave having a wavelength longer than a line length of each of the one-wavelength loop-shaped strip line resonators can be resonated. Finally, the microwave is transferred to the one-wavelength loop-shaped strip line resonator arranged in the final stage. Thereafter, the microwave is transferred from the second coupling point of the one-wavelength loop-shaped strip line resonator arranged in the final stage to the third coupling point of the one-wavelength loop-shaped strip line resonator arranged in the first stage. Thereafter, the microwave is resonated and filtered according to the second resonance mode in each of the one-wavelength loop-shaped strip line resonators. In this case, the microwave is influenced by the open-end transmission lines connected to the third and fourth coupling points. Finally, the microwave is transferred to the one-wavelength loop-shaped strip line resonator arranged in the final stage. Thereafter, the microwave is output from the fourth coupling point of the one-wavelength loop-shaped strip line resonator arranged in the final stage. Accordingly, attenuation characteristics of a microwave in the neighborhood of a passband of the microwave can be improved because the microwave is resonated and filtered two times in each of the one-wavelength loop-shaped strip line resonators. Also, because the open-end transmission lines influence the microwave, a small sized filter can be manufactured. It is preferred that the one-wavelength loop-shaped strip line resonators be respectively in a rectangular shape, the one-wavelength loop-shaped strip line resonators respectively have two first parallel lines longer than 90 degrees in electric length and two second parallel lines shorter than 90 degrees in electric length, the first and fourth coupling points be placed at the same first parallel line of each of the one-wavelength loop-shaped strip line resonators, the second and third coupling points be placed at the other first parallel line of each of the one-wavelength loop-shaped strip line resonators, and the first and second parallel coupling lines be formed of the second parallel lines of each of the one-wavelength loop-shaped strip line resonators. In the above configuration, because the fourth coupling point equivalent to a midpoint between the first and second coupling points is far from the second parallel coupling line and because the third coupling point equivalent to a midpoint between the first and second coupling points is far from the first parallel coupling line, a pair of notches surrounding a passband of the microwave resonated according to the first resonance mode can be formed, and the attenuation characteristics of the microwave can be enhanced. Also, because the second coupling point equivalent to a midpoint between the third and fourth coupling points is far from the second parallel coupling line and because the first coupling point equivalent to a midpoint between the third and fourth coupling points is far from the first parallel coupling line, the notches surrounding the passband of the microwave resonated according to the second resonance mode can be deepened, and the attenuation characteristics of the microwave can be moreover enhanced. The present invention also provides a strip line filter for resonating and filtering a microwave signal, comprising: a series of one-wavelength loop-shaped strip line resonators respectively having a uniform line impedance for respectively resonating and filtering a microwave signal in a first resonance mode in which electric voltages at both a first coupling point and a second coupling point spaced 180 degrees in electric length apart from the first coupling point are maximized and respectively resonating and filtering the microwave signal in a second resonance mode in which electric voltages at both a third coupling point spaced 90 degrees in electric length apart from the first coupling point and a fourth coupling point spaced 180 degrees in electric length apart from the third coupling point are maximized, each of the resonators having a first coupling line (L 2 ) between the first and third coupling points and a second coupling line (L 2 ) between the second and fourth coupling points, a microwave inputting element for inputting a microwave signal to the first coupling point of the resonator in a first stage; Wherein the second coupling line of the resonator in an N-th stage (N is an integral number) is electromagnetically coupled to the first parallel coupling line of another resonator arranged in an (N+1)-th stage to transfer the microwave signal between the resonator arranged in the N-th stage and the resonator arranged in the (N+1)-th stage; characterised by: a microwave outputting element for outputting the microwave signal resonating in the second resonance mode in resonator in the first stage; four open-ended transmission lines connected to the first, second, third and fourth coupling points of each of the resonators for electromagnetically influencing the microwave signals resonating therein, the open-ended transmission lines having the same electromagnetic characteristics; and an inter-stage coupling circuit for transferring the microwave signal resonating in the first resonance mode from the second coupling point of the resonator in the final stage to the fourth coupling point of the resonator in the final stage, the microwave signal transferred by the inter-stage coupling circuit resonating in the second resonance mode and being transferred by stages from the resonator of the final stage to the resonator arranged in the first stage, thereby to be filtered and output by the microwave outputting element. In the above configuration, the microwave resonated according to the first resonance mode by stages is transferred to the one-wavelength loop-shaped strip line resonator arranged in the final stage, in the same manner. Thereafter, the microwave is transferred from the second coupling point to the fourth coupling point of the one-wavelength loop-shaped strip line resonator arranged in the final stage. Thereafter, the microwave is resonated and filtered according to the second resonance mode in each of the one-wavelength loop-shaped strip line resonators, and transferred from the one-wavelength loop-shaped strip line resonator arranged in the final stage to the one-wavelength loop-shaped strip line resonator arranged in the first stage. In this case, the microwave is influenced by the open-end transmission lines connected to the third and fourth coupling points. Thereafter, the microwave is output from the third coupling point of the one-wavelength loop-shaped strip line resonator arranged in the first stage. Accordingly, attenuation characteristics of a microwave in the neighborhood of a passband of the microwave can be improved because the microwave is resonated and filtered two times in each of the one-wavelength loop-shaped strip line resonators. Also, because the open-end transmission lines influence the microwave, a small sized filter can be manufactured. The features and advantages of the present invention will be apparent from the following description of exemplary embodiments and the accompanying drawings, in which: Fig. 1 is a plan view of a conventional dual mode strip-line filter; Fig. 2A is a sectional view taken generally along the line II-II of Fig. 1; Fig. 2B is another sectional view taken generally along the line II-II of Fig. 1 according to a modification; Fig. 3 is a plan view of another conventional dual mode strip-line filter; Fig. 4 is an oblique view of a conventional dual mode resonator; Fig. 5 is a plan view of a strip-line filter described for reference; Fig. 6 is a plan view of a strip-line filter according to a modification of the filter of Figure 5; Fig. 7 is a plan view of another strip-line filter described for reference; Fig. 8 is a plan view of a strip-line filter according to a modification of the filter of Figure 7; Fig. 9 is a plan view of another strip-line filter described for reference; Fig. 10 is a plan view of another strip-line filter described for reference; Fig. 11 is a plan view of a strip-line filter according to a modification of the filter of Figure 10; Fig. 12 is a plan view of a strip-line filter according to a modification of the filter of Figure 10; Fig. 13 is a plan view of a strip line filter according to a modification of the filter of Figure 10; Fig. 14 is a plan view of a strip-line filter according to a modification of the filter of Figure 10; Fig. 15 is a plan view of another strip-line filter described for reference; Fig. 16 is a plan view of a strip line filter according to a modification of the filter of Figure 15; Fig. 17 is a plan view of a strip-line filter according to a modification of the filter of Figure15; Fig. 18 is a plan view of a strip-line filter according to a modification of the filter of Figure 15; Fig. 19 is a plan view of a strip-line filter according to a modification of the filter of Figure 15; Fig. 20 is a plan view of a strip-line filter according to a first embodiment of the present invention; Fig. 21 shows frequency characteristics of a microwave output from the strip-line filter shown in Fig. 20; Fig. 22 is a plan view of a strip-line filter according to a first modification of the first embodiment; Fig. 23 is a plan view of a strip-line filter according to a second modification of the first embodiment; Fig. 24 is a plan view of a strip-line filter according to a third modification of the first embodiment; Fig. 25 is a plan view of a strip-line filter according to a fourth modification of the first embodiment; Fig. 26 is a plan view of a strip-line filter according to a second embodiment; and Figs. 27 to 30 are respectively a plan to view of a strip-line filter according to a modification of the second embodiment. Preferred embodiments of a strip-line filter according to the present invention are described with reference to drawings. Fig. 5 is a plan view of a strip-line filter described for reference. As shown in Fig. 5, a strip-line filter 51 comprises an upper-stage filter 52a and a lower-stage filter 52b coupled to the upper-stage filter 52a through a parallel coupling space S1 in electromagnetic coupling. The upper-stage filter 52a comprises a first input terminal 53 excited by a first signal (or a first microwave) having a first resonance frequency f1, a second input terminal 54 excited by a second signal (or a second microwave) having a second resonance frequency f2, an upper-stage resonator 55 in which the first and second signals are resonated, a first input transmission line 56 connecting the first input terminal 53 with a coupling point A of the resonator 55 to couple the first input terminal 53 to the resonator 55, and a second input transmission line 57 connecting the second input terminal 54 with a coupling point C of the resonator 55 to couple the second input terminal 54 to the resonator 55. The lower-stage filter 52b comprises a lower-stage resonator 58 in which the first and second signals are resonated, a first output terminal 59 from which the first signal is output, a second output terminal 60 from which the second signal is output, a first output transmission line 61 connecting the first output terminal 59 with a coupling point F of the resonator 58 to couple the first output terminal 59 to the resonator 58, and a second output transmission line 62 connecting the second output terminal 60 with a coupling point H of the resonator 58 to couple the second output terminal 60 to the resonator 58. The shape of the upper-stage resonator 55 is the same as that of the lower-stage resonator 58. The upper-stage resonator 55 comprises a one-wavelength square-shaped strip line resonator 63 having a uniform characteristic line impedance, a pair of first open-end transmission lines 64a, 64b connected to coupling points A and B of the resonator 63 for electromagnetically influencing the first signal, and a pair of second open-end transmission lines 65c, 65d connected to coupling points C and D of the resonator 63 for electromagnetically influencing the second signal. The one-wavelength square-shaped strip line resonator 63 represents a one-wavelength loop-shaped strip line resonator. The first open-end transmission lines 64a, 64b have the same electromagnetic characteristics, and the second open-end transmission lines 65c, 65d have the same electromagnetic characteristics which differ from those of the first open-end transmission lines 64a, 64b. The coupling points A,C,B and D are placed at four corners of the line resonator 63 in that order. In detail, the coupling point B is spaced 180 degrees in the electric length apart from the coupling point A. The coupling point C is spaced 90 degrees in the electric length apart from the coupling point A. The coupling point D is spaced 180 degrees in the electric length apart from the coupling point C. The lower-stage resonator 58 comprises a one-wavelength square-shaped strip line resonator 66 having the same uniform characteristic line impedance as that of the resonator 63, first open-end transmission lines 64e, 64f connected to coupling points E and F of the resonator 66, and second open-end transmission lines 65g, 65h connected to coupling points G and H of the resonator 66. The one-wavelength square-shaped strip line resonator 66 represents a one-wavelength loop-shaped strip line resonator. The first open-end transmission lines 64e, 64f have the same electromagnetic characteristics as those of the first open-end transmission lines 64a, 64b, and the second open-end transmission lines 65g, 65h have the same electromagnetic characteristics as those of the second open-end transmission lines 65c, 65d. The coupling points E,G,F and H are placed at four corners of the line resonator 66 and are spaced 90 degrees in the electric length in that order. A straight strip line of the resonator 63 between the coupling points B and D faces a straight strip line of the resonator 66 between the coupling points G and E in parallel through the parallel coupling space S1 to arrange the first open-end transmission lines 64a, 64b of the resonator 55 symmetrically to the first open-end transmission lines 64e, 64f of the resonator 58 with respect to a central point of the parallel coupling space S1. In the above configuration, when the first input terminal 53 is excited by microwaves having various frequencies in which a first signal having a resonance frequency f1 (or a resonance wavelength λ 1 ) is included, the first input terminal 53 is coupled to the coupling point A of the resonator 63 through the first input transmission line 56, and the microwaves including the first signal are transferred to the upper-stage resonator 55. Thereafter, the first signal is selectively resonated in the upper-stage resonator 55 at the resonance frequency f1 according to a first resonance mode. The resonance frequency f1 selectively resonated is determined by a characteristic impedance of the line resonator 63 and electromagnetic characteristics of the first open-end transmission lines 64a, 64b. In this case, a half-wavelength λ 1 /2 corresponding to the resonance frequency f1 is longer than a line length between the coupling points A and B because of the electromagnetic characteristics of the first open-end transmission lines 64a, 64b. Thereafter, electric voltages at the coupling points A and B reach a maximum value, and electric currents at the coupling points C and D reach a maximum value. That is, electric voltages at the coupling points C and D are zero. Thereafter, the first signal resonated is transferred to the lower-stage resonator 58 through the parallel coupling space S1 because the upper-stage filter 52a is coupled to the lower-stage filter 52b. Thereafter, the first signal is selectively resonated in the resonator 58 at the resonance frequency f1 according to the first resonance mode. Electric voltages at the coupling points E and F reach a maximum value, and electric currents at the coupling points G and H reach a maximum value. That is, electric voltages at the coupling points G and H are zero. Thereafter, the first signal resonated in the resonator 58 is transferred to the first output terminal 59 through the first output transmission line 61 because the electric voltage of the coupling point F is maximized. In contrast, when the second input terminal 54 is excited by microwaves having various frequencies in which a second signal having a resonance frequency f2 (or a resonance wavelength λ 2 ) is included, the second input terminal 54 is coupled to the coupling point C of the resonator 55 through the second input transmission line 57, and the microwaves including the second signal are transferred to the resonator 55. Thereafter, the second signal is selectively resonated in the resonator 55 at the resonance frequency f2 according to a second resonance mode. The resonance frequency f2 selectively resonated is determined by a characteristic impedance of the line resonator 63 and electromagnetic characteristics of the second open-end transmission lines 65c, 65d. In this case, a half-wavelength λ 2 /2 corresponding to the resonance frequency f2 is longer than a line length between the coupling points C and D because of the electromagnetic characteristics of the second open-end transmission lines 65c, 65d. Thereafter, electric voltages at the coupling points C and D reach a maximum value, and electric currents at the coupling points A and B reach a maximum value. That is, electric voltages at the coupling points A and B are zero. Thereafter, the second signal resonated is transferred to the resonator 66 through the parallel coupling space S1, and the second signal is selectively resonated in the resonator 66 at the resonance frequency f2 according to the second resonance mode. Electric voltages at the coupling points G and H reach a maximum value, and electric currents at the coupling points E and F reach a maximum value. That is, electric voltages at the coupling points E and F are zero. Thereafter, the second signal resonated in the resonator 66 is transferred to the second output terminal 60 through the second output transmission line 62 because the electric voltage of the coupling point H is maximized. A first phase of the first signal resonated according to the first resonance mode and another phase of the second signal resonated according to the second resonance mode are orthogonal to each other in each of the upper-stage and the lower-stage resonators 55, 58. Therefore, even though an electric voltage of the first signal (or the second signal) is maximized at a first point, because an electric voltage of the first signal (or the second signal) at a second point spaced 90 degrees in the electric length apart from the first point is zero, the first signal does not couple to the second signal at the second point at which an electric voltage of the second signal (or the first signal) is maximized. In other words, the first and second signals having different frequencies f1, f2 coexist independently in the strip-line filter 51. Accordingly, the upper-stage and lower-stage resonators 55, 58 of the strip-line filter 51 can function as resonators for the first and second signals having different resonance frequencies, and the strip-line filter 51 can function as a filter for the first and second signals. Also, because the half-wavelength λ 1 /2 corresponding to the resonance frequency f1 is longer than a line length between the coupling points A and B and because the half-wavelength λ 2 /2 corresponding to the resonance frequency f2 is longer than a line length between the coupling points C and D, the resonance frequencies f1, f2 can be lower than an original resonance frequency f0 corresponding to a wavelength λ o of which a half value λ o /2 is equal to the line length between the coupling points A and B (that is, the line length between the coupling points C and D). In other words, sizes of the resonators 63, 66 can be smaller than that of a resonator in which any open-end transmission lines do not provided, so that the strip-line filter 51 can be manufactured in a small size. Also, because a straight strip line of the resonator 63 and another straight strip line of the resonator 66 arranged in parallel to each other are coupled to each other through the parallel coupling space S1, the upper-stage resonator 63 and the lower-stage resonator 66 can be arranged closely to each other. Therefore, the strip-line filter 51 can be manufactured in a small size. Also, the resonance frequency f1 can be arbitrarily set by setting the first open-end transmission lines 64a, 64b, 64e and 64f to a prescribed length and the resonance frequency f2 can be arbitrarily set by setting the second open-end transmission lines 65c, 65d, 65g and 65h. Also, the resonance frequency f1 can be accurately adjusted by trimming or overlaying end portions of the first open-end transmission lines 64a, 64b, 64e and 64f, and the resonance frequency f2 can be accurately adjusted by trimming or overlaying end portions of the second open-end transmission lines 65c, 65d, 65g and 65h. Also, because the open-end transmission lines are formed of strip lines, the strip-line filter 51 can be manufactured in a plane shape. Fig. 6 is a plan view of a strip-line filter which is a modification of strip-line filter 51. As shown in Fig. 6, a strip-line filter 67 comprises an upper-stage filter 68a and a lower-stage filter 68b coupled to the upper-stage filter 68a through a parallel coupling space S2 in electromagnetic coupling. The upper-stage filter 68a comprises the first input terminal 53, the second input terminal 54 excited by a third signal (or a third microwave) having an original resonance frequency f0, an upper-stage resonator 69 in which the first and third signals are resonated, the first input transmission line 56 connecting the first input terminal 53 with a coupling point A of the resonator 69, and the second input transmission line 57 connecting the second input terminal 54 with a coupling point C of the resonator 69. The lower-stage filter 68b comprises a lower-stage resonator 70 in which the first and third signals are resonated, the first output terminal 59, the second output terminal 60 from which the third signal is output, the first output transmission line 61 connecting the first output terminal 59 with a coupling point F of the resonator 70, and the second output transmission line 62 connecting the second output terminal 60 with a coupling point H of the resonator 70. The upper-stage resonator 69 comprises the one-wavelength rectangular-shaped strip line resonator 63 and the first open-end transmission lines 64a, 64b. The lower-stage resonator 70 comprises the one-wavelength rectangular-shaped strip line resonator 66 and the first open-end transmission lines 64e, 64f. A straight strip line of the resonator 63 between the coupling points B and D faces a straight strip line of the resonator 66 between the coupling points G and E in parallel through the parallel coupling space S2 to arrange the first open-end transmission lines 64a, 64b of the resonator 69 symmetrically to the first open-end transmission lines 64e, 64f of the resonator 70 with respect to a central point of the parallel coupling space S2. In the above configuration, the first signal is resonated and filtered in the strip-line filter 67 in the same manner as in the strip-line filter 51. In contrast, when the second input terminal 54 is excited by microwaves having various frequencies in which a third signal having an original resonance frequency f0 (or an original resonance wavelength λ o ) is included, the third signal is selectively resonated in the resonator 69 at the original resonance frequency f0 according to an original resonance mode. The original resonance frequency f0 selectively resonated is determined by the characteristic impedance of the line resonator 63. Therefore, the original resonance frequency f0 is higher than the resonance frequency f1. Thereafter, the third signal is transferred to the lower-stage resonator 70 and is resonated and filtered. Thereafter, the third signal is output from the second output terminal 60. Accordingly, the third signal which has an original resonance frequency f0 determined by the characteristic impedance of the line resonator 63 can be resonated and filtered in the strip-line filter 67 in addition to the resonance and filtering of the first signal. Also, frequency adjustment of the first signal can be easily performed, and a small sized filter for filtering the first and third signals can be manufactured in a plane shape. In the strip-line filters shown in Figs. 5 and 6, the open-end transmission lines are integrally formed with the line resonators 63, 66 according to a pattern formation. However, it is applicable that the open-end transmission lines be formed after the line resonators 63, 66 are formed. Next, a second strip-line filter is described with reference to Figs. 7 and 8. Fig. 7 is a plan view of another strip-filter described for reference. As shown in Fig. 7, a strip-line filter 71 comprises the upper-stage filter 52a and a lower-stage filter 52c coupled to the upper-stage filter 52a through a parallel coupling space S3 in electromagnetic coupling. The lower-stage filter 52c comprises a lower-stage resonator 72 in which the first and second signals having the resonance frequencies f1, f2 are resonated, the first output terminal 59, the second output terminal 60, the first output transmission line 61 connecting the first output terminal 59 with a coupling point H of the resonator 72, and the second output transmission line 62 connecting the second output terminal 60 with a coupling point F of the resonator 72. The lower-stage resonator 72 comprises the one-wavelength rectangular-shaped strip line resonator 66, a pair of first open-end transmission lines 64g, 64h connected to coupling points G and H of the resonator 66, and a pair of second open-end transmission lines 65e, 65f connected to coupling points E and F of the resonator 66. The first open-end transmission lines 64g, 64h have the same electromagnetic characteristics as those of the first open-end transmission lines 64a, 64b, and the second open-end transmission lines 65e, 65f have the same electromagnetic characteristics as those of the second open-end transmission lines 65c, 65d. The coupling points E,F,G and H are spaced 90 degrees in the electric length apart in that order. A straight strip line of the resonator 63 between the coupling points B and D faces a straight strip line of the resonator 66 between the coupling points G and E in parallel through the parallel coupling space S3 to arrange the first open-end transmission lines 64a, 64b of the resonator 55 symmetrically to the first open-end transmission lines 64g, 64h of the resonator 72 with respect to a central axis of the parallel coupling space S3. In the above configuration, a first signal having the resonance frequency f1 (or the resonance wavelength λ 1 ) is resonated and filtered in the upper-stage filter 52a in the same manner as in the strip-line filter 51. That is, the resonance frequency f1 is determined by the characteristic impedance of the line resonator 63 and the electromagnetic characteristics of the first open-end transmission lines 64a, 64b, so that the half-wavelength λ 1 /2 corresponding to the resonance frequency f1 is longer than a line length between the coupling points A and B. Thereafter, the first signal is transferred to the lower-stage filter 52c through the parallel coupling space S3. Thereafter, the first signal is selectively resonated in the resonator 72 at the resonance frequency f1 according to the first resonance mode. Electric voltages at the coupling points G and H reach a maximum value, and electric currents at the coupling points E and F reach a maximum value. That is, electric voltages at the coupling points E and F are zero. Thereafter, the first signal resonated in the resonator 72 is transferred to the first output terminal 59 through the first output transmission line 61 because the electric voltage of the coupling point H is maximized. In contrast, a second signal having the resonance frequency f2 (or the resonance wavelength λ 2 ) is resonated and filtered in the upper-stage filter 52a in the same manner as in the first embodiment. That is, the resonance frequency f2 is determined by the characteristic impedance of the line resonator 63 and the electromagnetic characteristics of the second open-end transmission lines 65c, 65d, so that the half-wavelength λ 2 /2 corresponding to the resonance frequency f2 is longer than a line length between the coupling points C and D. Thereafter, the second signal is transferred to the lower-stage filter 52c through the parallel coupling space S3. Thereafter, the second signal is selectively resonated in the resonator 72 at the resonance frequency f2 according to the second resonance mode. Electric voltages at the coupling points E and F reach a maximum value, and electric currents at the coupling points G and H reach a maximum value. That is, electric voltages at the coupling points G and H are zero. Thereafter, the second signal resonated in the resonator 72 is transferred to the second output terminal 60 through the second output transmission line 62 because the electric voltage of the coupling point F is maximized. The first phase of the first signal resonated according to the first resonance mode and the second phase of the second signal resonated according to the second resonance mode are orthogonal to each other in each of the upper-stage and the lower-stage resonators 55, 72. Therefore, even though an electric voltage of the first signal (or the second signal) is maximized at a first point, because an electric voltage of the first signal (or the second signal) at a second point spaced 90 degrees in the electric length apart from the first point is zero, the first signal does not couple to the second signal at the second point at which an electric voltage of the second signal (or the first signal) is maximized. In other words, the first and second signals having different frequencies f1, f2 coexist independently in the strip-line filter 71. Accordingly, the upper-stage and lower-stage resonators 55, 72 of the strip-line filter 71 can function as resonators for the first and second signals having different resonance frequencies, and the strip-line filter 71 can function as a filter for the first and second signals. Also, because the half-wavelength λ 1 /2 corresponding to the resonance frequency f1 is longer than a line length between the coupling points A and B and because the half-wavelength λ 2 /2 corresponding to the resonance frequency f2 is longer than a line length between the coupling points C and D, the resonance frequencies f1, f2 can be lower than an original resonance frequency f0 corresponding to a wavelength λ o of which a half value λ o /2 is equal to the line length between the coupling points A and B (that is, the line length between the coupling points C and D). In other words, sizes of the resonators 63, 66 can be smaller than that of a resonator in which any open-end transmission lines do not provided, so that the strip-line filter 71 can be manufactured in a small size. Also, because a straight strip line of the resonator 63 and another straight strip line of the resonator 66 arranged in parallel to each other are coupled to each other through the parallel coupling space S3, the upper-stage resonator 63 and the lower-stage resonator 66 can be arranged closely to each other. Therefore, the strip-line filter 71 can be manufactured in a small size. Also, the resonance frequency f1 can be arbitrarily set by setting the first open-end transmission lines to a prescribed line length, and the resonance frequency f2 can be arbitrarily set by setting the second open-end transmission lines to a prescribed line length. Also, the resonance frequency f1 can be accurately adjusted by trimming or overlaying end portions of the first open-end transmission lines, and the resonance frequency f2 can be accurately adjusted by trimming or overlaying end portions of the second open-end transmission lines. Also, because all of the open-end transmission lines are formed of strip lines, the strip-line filter 71 can be manufactured in a plane shape. Fig. 8 is a plan view of a strip-line filter according to a modification of strip-line filter 71. As shown in Fig. 8, a strip-line filter 81 comprises the upper-stage filter 68a and a lower-stage filter 68c coupled to the upper-stage filter 68a through a parallel coupling space S4 in electromagnetic coupling. The lower-stage filter 68c comprises a lower-stage resonator 82 in which the first and third signals are resonated, the first output terminal 59, the second output terminal 60, the first output transmission line 61 connecting the first output terminal 59 with a coupling point H of the resonator 82, and the second output transmission line 62 connecting the second output terminal 60 with a coupling point F of the resonator 82. The lower-stage resonator 82 comprises the one-wavelength rectangular-shaped strip line resonator 66 and the first open-end transmission lines 64g, 64h connected to coupling points G and H of the resonator 66. The coupling points E,F,G and H are spaced 90 degrees in the electric length apart in that order. A straight strip line of the resonator 63 between the coupling points B and D faces a straight strip line of the resonator 66 between the coupling points G and E in parallel through the parallel coupling space S4 to arrange the first open-end transmission lines 64a, 64b of the resonator 69 symmetrically to the first open-end transmission lines 64g, 64h of the resonator 82 with respect to a central axis of the parallel coupling space S4. In the above configuration, a first signal having the resonance frequency f1 resonated and filtered in the upper-stage filter 68a in the same manner as in the strip-line filter 51 is transferred to the lower-stage filter 68c through the parallel coupling space S4. Thereafter, the first signal is selectively resonated in the resonator 82 at the resonance frequency f1 according to the first resonance mode. Electric voltages at the coupling points G and H reach a maximum value, and electric voltages at the coupling points E and F are zero. Thereafter, the first signal resonated in the resonator 82 is transferred to the first output terminal 59 through the first output transmission line 61 because the electric voltage of the coupling point H is maximized. In contrast, a third signal having the original resonance frequency f0 resonated and filtered in the upper-stage filter 68a in the same manner as in the first embodiment is transferred to the lower-stage filter 68c through the parallel coupling space S4. Thereafter, the third signal is selectively resonated in the resonator 82 at the resonance frequency f0 according to the third resonance mode. Electric voltages at the coupling points E and F reach a maximum value, and electric voltages at the coupling points G and H are zero. Thereafter, the third signal resonated in the resonator 82 is transferred to the second output terminal 60 through the second output transmission line 62 because the electric voltage of the coupling point F is maximized. Accordingly, the third signal which has the original resonance frequency f0 determined by the characteristic impedance of the line resonator 63 can be resonated and filtered in the strip-line filter 67 in addition to the resonance and filtering of the first signal. Also, frequency adjustment of the first signal can be easily performed, and a small sized filter for filtering the first and third signals can be manufactured in a plane shape. In the strip-line filters 71, 81 shown in Figs. 7 and 8, all of the open-end transmission lines are integrally formed with the line resonators 63, 66 according to a pattern formation. However, it is applicable that the open-end transmission lines be formed after the line resonators 63, 66 are formed. Fig. 9 is a plan view of a strip-line filter described for reference. As shown in Fig. 9, a strip-line filter 91 comprises an upper-stage filter 92a and a lower-stage filter 92b coupled to the upper-stage filter 92a through a parallel coupling space S5 in electromagnetic coupling. The upper-stage filter 92a comprises the first input terminal 53, the second input terminal 54, an upper-stage resonator 93 in which two propagating signals having the same resonance frequency fl are resonated, the first input transmission line 56, and the second input transmission line 57. The lower-stage filter 92b comprises a lower-stage resonator 94 in which the propagating signals are resonated, the first output terminal 59, the second output terminal 60, the first output transmission line 61, and the second output transmission line 62. The upper-stage resonator 93 comprises the one-wavelength rectangular-shaped strip line resonator 63 and four first open-end transmission lines 64a, 64b, 64c and 64d connected to the coupling points A to D of the resonator 63. The first open-end transmission lines 64a, 64b, 64c and 64d have the same electromagnetic characteristics. The lower-stage resonator 94 comprises the one-wavelength rectangular-shaped strip line resonator 66 and four first open-end transmission lines 64e, 64f, 64g and 64h connected to the coupling points E to H of the resonator 66. The first open-end transmission lines 64e, 64f, 64g and 64h have the same electromagnetic characteristics as those of the first open-end transmission lines 64a, 64b, 64c and 64d. A straight strip line of the resonator 63 between the coupling points B and D faces a straight strip line of the resonator 66 between the coupling points G and E in parallel through the parallel coupling space S5. In the above configuration, when the first input terminal 53 (or the second input terminal 54) is excited by microwaves having various frequencies in which a propagating signal S1 (or S2) having the resonance frequency fl is included, the microwaves including the propagating signal are transferred to the upper-stage resonator 93. Thereafter, the propagating signal is selectively resonated in the upper-stage resonator 93 at the resonance frequency f1 according to the first resonance mode. The resonance frequency f1 selectively resonated is determined by the characteristic impedance of the line resonator 63 and electromagnetic characteristics of the first open-end transmission lines 64a and 64b (or 64c and 64d). In this case, the half-wavelength λ 1 /2 corresponding to the resonance frequency f1 is longer than a line length between the coupling points A and B (or the coupling points C and D) of the line resonator 63 because of the electromagnetic characteristics of the first open-end transmission lines 64a and 64b (or 64c and 64d). Thereafter, electric voltages at the coupling points A and B (or the coupling points C and D) reach a maximum value, and electric voltages at the coupling points C and D (the coupling points A and B) are zero. Thereafter, the propagating signal resonated is transferred to the lower-stage resonator 94 through the parallel coupling space S5, and the propagating signal is selectively resonated in the resonator 94 at the resonance frequency f1 according to the first resonance mode. Electric voltages at the coupling points E and F (or the coupling points G and H) reach a maximum value, and electric voltages at the coupling points G and H (or the coupling points E and F) are zero. Thereafter, the propagating signal resonated in the resonator 94 is transferred to the first output terminal 59 (or the second output terminal 60) through the first output transmission line 61 (or the second output transmission line 62) because the electric voltage of the coupling point H (or the coupling point F) is maximized. Phases of the propagating signals S1 and S2 resonated according to the first resonance mode are orthogonal to each other in each of the upper-stage and the lower-stage resonators 93, 94. Therefore, even though an electric voltage of the propagating signal S1 is maximized at a first point, because an electric voltage of the propagating signal S1 at a second point spaced 90 degrees in the electric length apart from the first point is zero, the propagating signal S1 does not couple to the propagating signal S2 at the second point at which an electric voltage of the propagating signal S2 is maximized. In other words, the propagating signals S1 and S2 having the same frequency fl coexist independently in the strip-line filter 91. Accordingly, the upper-stage and lower-stage resonators 93, 94 of the strip-line filter 91 can function as resonators for the propagating signals having the same resonance frequency, and the strip-line filter 91 can function as a filter for the propagating signals. Also, because the half-wavelength λ 1 /2 corresponding to the resonance frequency f1 is longer than a line length between the coupling points A and B, the resonance frequency f1 can be lower than an original resonance frequency f0 corresponding to a wavelength λ o of which a half value λ o /2 is equal to the line length between the coupling points A and B. In other words, sizes of the resonators 93, 94 can be smaller than that of a resonator in which any open-end transmission lines do not provided, so that the strip-line filter 91 can be manufactured in a small size. Also, because a straight strip line of the resonator 63 and another straight strip line of the resonator 66 arranged in parallel to each other are coupled to each other through the parallel coupling space S5, the upper-stage resonator 63 and the lower-stage resonator 66 can be arranged closely to each other. Therefore, the strip-line filter 91 can be manufactured in a small size. Also, the resonance frequency f1 can be arbitrarily set by setting the first open-end transmission lines to a prescribed line length. Also, the resonance frequency f1 can be accurately adjusted by trimming or overlaying end portions of the first open-end transmission lines. Also, because all of the open-end transmission lines are formed of strip lines, the strip-line filter 91 can be manufactured in a plane shape. In case of the strip-line filters 51, 67, 71, 81 and 91 shown in Figs. 5 to 9, because the straight strip line of the resonator 63 (or 66) facing the straight strip line of the resonator 66 (or 63) has an electric length of 90 degrees, the coupling between the first-stage filter 52a, 68a or 92a and the second-stage filter 52b, 68b, 52c, 68c or 92b is strong. Therefore, in cases where the strip-line filter 51, 67, 71, 81 or 91 is utilized in a narrow passband, it is required to widen a distance between the first-stage filter and the second-stage filter. As a result, there is a drawback that it is difficult to lessen unnecessary couplings and make the strip-line filter small. This drawback is solved by the provision of strip-line filter 101 as described below. Fig. 10 is a plan view of another strip-line filter described for reference. As shown in Fig. 10, a strip-line filter 101 comprises an upper-stage filter 102a and a lower-stage filter 102b coupled to the upper-stage filter 102a through a parallel coupling space S6 in electromagnetic coupling. The upper-stage filter 102a comprises the first input terminal 53, the second input terminal 54, an upper-stage resonator 103 in which first and second signals are resonated, the first input transmission line 56 connecting the first input terminal 53 with a coupling point A of the resonator 103, and the second input transmission line 57 connecting the second input terminal 54 with a coupling point C of the resonator 103. The lower-stage filter 102b comprises a lower-stage resonator 104 in which the first and second signals are resonated, the first output terminal 59, the second output terminal 60, the first output transmission line 61 connecting the first output terminal 59 with a coupling point F of the resonator 104, and the second output transmission line 62 connecting the second output terminal 60 with a coupling point H of the resonator 104. The shape of the upper-stage resonator 103 is the same as that of the lower-stage resonator 104. The upper-stage resonator 103 comprises a one-wavelength rectangular-shaped strip line resonator 105 having a uniform characteristic line impedance, the first open-end transmission lines 64a, 64b connected to coupling points A and B of the resonator 105, and the second open-end transmission lines 65c, 65d connected to coupling points C and D of the resonator 105. The one-wavelength rectangular-shaped strip line resonator 105 represents a one-wavelength loop-shaped strip line resonator. The line resonator 105 is composed of two first parallel lines L1 and two second parallel lines L2 shorter than the lines Ll. The coupling points A,C,B and D are placed at the first parallel lines L1 of the line resonator 105 and are spaced 90 degrees in the electric length in that order. The lower-stage resonator 104 comprises a one-wavelength square-shaped strip line 106 having the same uniform characteristic line impedance as that of the resonator 105, the first open-end transmission lines 64e, 64f connected to coupling points E and F of the line resonator 106, and the second open-end transmission lines 65g, 65h connected to coupling points G and H of the line resonator 106. The one-wavelength rectangular-shaped strip line resonator 106 represents a one-wavelength loop-shaped strip line resonator. The coupling points E,G,F and H are placed at the first parallel lines L1 of the line resonator 106 and are spaced 90 degrees in the electric length in that order. A second parallel line L2 of the resonator 105 closely faces a second parallel line L2 of the resonator 106 in parallel through the parallel coupling space S6 to arrange the first open-end transmission lines 64a, 64b of the resonator 103 symmetrically to the first open-end transmission lines 64e, 64f of the resonator 104 with respect to a central point of the parallel coupling space S6. The second parallel line L2 of the resonator 105 closely facing the resonator 106 is called a parallel coupling line L2, and the second parallel line L2 of the resonator 106 closely facing the resonator 105 is called another parallel coupling line L2. In the above configuration, electric lengths of the parallel coupling lines L2 of the resonators 105, 106 are respectively less than 90 degrees. Therefore, the coupling between the first-stage filter 102a and the second-stage filter 102b does not becomes strong even though the first-stage filter 102a is arranged closely to the second-stage filter 102b. The operation in the strip-line filter 101 is the same as that in the strip-line filter 51, so that the description of the operation is omitted. Accordingly, the first-stage filter 102a can be arranged closely to the second-stage filter 102b, and unnecessary couplings and area occupied by the strip-line filter 101 can be reduced in addition to effects obtained in strip-line filter 51. Fig. 15 is a plan view of another strip-line filter described for reference. As shown in Fig. 15, a strip-line filter 111 comprises an upper-stage filter 112a and a lower-stage filter 112b coupled to the upper-stage filter 112a through the parallel coupling space S6 in electromagnetic coupling. The upper-stage filter 102a comprises the first input terminal 53, the second input terminal 54, the upper-stage resonator 103, a first input parallel coupling strip line 113 for coupling the first input terminal 53 to the coupling point A of the upper-stage resonator 103, and a second input parallel coupling strip line 114 for coupling the second input terminal 54 to the coupling point C of the upper-stage resonator 103. The lower-stage filter 102b comprises the lower-stage resonator 104, the first output terminal 59, the second output terminal 60, a first output parallel coupling strip line 115 for coupling the first output terminal 59 to the coupling point F of the lower-stage resonator 104, a second output parallel coupling strip line 116 for coupling the second output terminal 60 to the coupling point H of the lower-stage resonator 104. In the above configuration, when the first input terminal 53 is excited by microwaves having various frequencies in which a first signal having the resonance frequency f1 is included, the first input parallel coupling strip line 113 is coupled to a first parallel line L1 of the line resonator 105, and the microwaves are transferred to the upper-stage resonator 103. Thereafter, the first signal is resonated and filtered in the upper-stage resonator 103 and the lower-stage resonator 104 in the same manner as in the first embodiment. Thereafter, the first output parallel coupling strip line 115 is coupled to a first parallel line L1 of the line resonator 106. Therefore, the first signal is output to the first output terminal 59. In contrast, when the second input terminal 54 is excited by microwaves having various frequencies in which a second signal having the resonance frequency f2 is included, the second input parallel coupling strip line 114 is coupled to another first parallel line L1 of the line resonator 105, and the microwaves are transferred to the upper-stage resonator 103. Thereafter, the second signal is resonated and filtered in the upper-stage resonator 103 and the lower-stage resonator 104 in the same manner as in the first embodiment. Thereafter, the second output parallel coupling strip line 116 is coupled to another second parallel line L1 of the line resonator 106. Therefore, the second signal is output to the second output terminal 60. Accordingly, because the input and output parallel coupling strip lines 113 to 116 are utilized to input and output the first and second signals, input and output elements of the strip-line filter 111 can be downsized and simplified, in addition to effects obtained in strip-line filter 101. Each of the strip line filters described above is formed of two-stage filters. However, the number of stages in the strip-line filter is not limited to two stages. That is, a multi-stage type strip-line filter can be useful. Fig. 20 is a plan view of a strip-line filter according to a first embodiment of the present invention, and Fig. 21 shows frequency characteristics of a microwave output from the strip-line filter shown in Fig. 20. As shown in Fig. 20, a strip-line filter 201 comprises an upper-stage filter 202a, a lower-stage filter 202b coupled to the upper-stage filter 202a through the parallel coupling space S6 in electromagnetic coupling, and an inter-stage coupling circuit 203 connecting a coupling point H of the lower-stage filter 202b to a coupling point C of the upper-stage filter 202a. The upper-stage filter 202a comprises an input terminal 204 excited by microwaves, an upper-stage resonator 205 for selectively resonating a propagating signal included in the microwaves, an input coupling circuit 206 for coupling the input terminal 204 to a coupling point A of the resonator 205. The lower-stage filter 202b comprises a lower-stage resonator 207 for selectively resonating the propagating signal, an output terminal 208 for outputting the propagating signal, and an output coupling circuit 209 for coupling the output terminal 208 to a coupling point F of the resonator 207. The shape of the upper-stage resonator 205 is the same as that of the lower-stage resonator 207. The upper-stage resonator 205 comprises the one-wavelength rectangular-shaped strip line resonator 105 and the four open-end transmission lines 64a to 64d connected to coupling points A to D of the resonator 105. The coupling points A,C,B and D are placed at the first parallel lines L1 of the line resonator 105 and are spaced 90 degrees in the electric length in that order. The lower-stage resonator 207 comprises the one-wavelength rectangular-shaped strip line resonator 106 and the four open-end transmission lines 64f to 64i connected to coupling points F to I of the resonator 106. The coupling points I,G,H and F are placed at the first parallel lines L1 of the line resonator 106 and are spaced 90 degrees in electric length in that order. A midpoint E placed in the middle of the parallel coupling line L2 of the line resonator 105 is defined, and a midpoint K placed in the middle of the parallel coupling line L2 of the line resonator 106 is defined. An electric length between the coupling point D and the midpoint E, an electric length between the coupling point B and the midpoint E, an electric length between the coupling point I and the midpoint K and an electric length between the coupling point G and the midpoint K are the same value. In the above configuration, when the input terminal 204 is excited by microwaves having various frequencies in which a propagating signal having a resonance frequency f1 (corresponding to a resonance wavelength λ 1 ) is included, the input terminal 204 is coupled to a first parallel line L1 of the line resonator 105, and the microwaves are transferred to the upper-stage resonator 205. Thereafter, the propagating signal is selectively resonated in the upper-stage resonator 205 at the resonance frequency f1 according to a first resonance mode. The resonance frequency f1 selectively resonated is determined by a characteristic impedance of the line resonator 105 and electromagnetic characteristics of the open-end transmission lines 64a and 64b. In this case, a half-wavelength λ 1 /2 corresponding to the resonance frequency f1 is longer than a line length between the coupling points A and B because of the electromagnetic characteristics of the first open-end transmission lines 64a and 64b. Thereafter, electric voltages at the coupling points A and B reach a maximum value, and electric currents at the coupling points C and D reach a maximum value. That is, electric voltages at the coupling points C and D are zero. Thereafter, the propagating signal resonated is transferred to the lower-stage resonator 207 through the parallel coupling space S6 because the upper-stage filter 202a is coupled to the lower-stage filter 202b, and the propagating signal is selectively resonated in the resonator 207 at the resonance frequency f1 according to the first resonance mode. Electric voltages at the coupling points H and I reach a maximum value, and electric currents at the coupling points F and G reach a maximum value. That is, electric voltages at the coupling points F and G are zero. In this case, because the coupling point D placed in the middle of the coupling points A and B is outside the parallel coupling line L2 of the line resonator 105 and because the coupling point G placed in the middle of the coupling points H and I is outside the parallel coupling line L2 of the line resonator 106, as shown in Fig. 21, a pair of notches occur in the neighborhood of a passband of the microwaves. Thereafter, the propagating signal resonated in the resonator 207 is transferred from the coupling point H to the coupling point C through the inter-stage coupling circuit 203 because the electric voltage of the coupling point H is maximized. Thereafter, the propagating signal is selectively resonated in the upper-stage resonator 205 at the resonance frequency f1 according to a second resonance mode orthogonal to the first resonance mode. The resonance frequency f1 selectively resonated is determined by the characteristic impedance of the line resonator 105 and electromagnetics characteristics of the open-end transmission lines 64c and 64d. Electric voltages at the coupling points C and D reach a maximum value, and electric voltages at the coupling points A and B are zero. Thereafter, the propagating signal resonated is again transferred to the lower-stage resonator 207 through the parallel coupling space S6, and the propagating signal is selectively resonated in the resonator 207 at the resonance frequency f1 according to the second resonance mode. Electric voltages at the coupling points F and G reach a maximum value, and electric voltages at the coupling points H and I are zero. In this case, because the coupling point B placed in the middle of the coupling points C and D is outside the parallel coupling line L2 of the line resonator 105 and because the coupling point I placed in the middle of the coupling points F and G is outside the parallel coupling line L2 of the line resonator 106, as shown in Fig. 21, the notches occurring in the neighborhood of the passband of the microwaves are deepened. Thereafter, the propagating signal is output to the output terminal 208 through the output coupling circuit 209 because the electric voltage at the coupling point F is maximized. Accordingly, because a pair of notches surrounding the passband of microwaves occur and is deepened in the strip-line filter 201, a filter having excellent attenuation characteristics can be manufactured even though the number of stages in the filter is low. Also, because the half-wavelength λ 1 /2 corresponding to the resonance frequency f1 is longer than a line length between the coupling points A and B, the resonance frequency f1 can be lower than an original resonance frequency f0 corresponding to a wavelength λ o of which a half value λ o /2 is equal to the line length between the coupling points A and B (that is, the line length between the coupling points C and D). In other words, sizes of the line resonators 105, 106 can be smaller than that of a resonator in which any open-end transmission lines are provided, so that the strip-line filter 201 can be manufactured in a small size. Also, because electric lengths of the parallel coupling lines L2 of the resonators 105, 106 are respectively less than 90 degrees, the first-stage filter 202a can be arranged closely to the second-stage filter 202b, and unnecessary couplings and area occupied by the strip-line filter 201 can be reduced. Also, the resonance frequency f1 can be arbitrarily set by setting the open-end transmission lines to a prescribed line length. Also, the resonance frequency f1 can be accurately adjusted by trimming or overlaying end portions of the open-end transmission lines. Also, because all of the open-end transmission lines are formed of strip lines and because the coupling circuits 203, 206 and 209 can be respectively formed of a pair of parallel coupling strip-lines, the strip-line filter 201 can be manufactured in a plane shape. Next, a first modification of the first embodiment is described with reference to Fig. 22. Fig. 22 is a plan view of a strip-line filter according to a first modification of the first embodiment. As shown in Fig. 22, a strip-line filter 221 comprises an upper-stage filter 222a, a lower-stage filter 222b coupled to the upper-stage filter 222a through the parallel coupling space S6 in electromagnetic coupling, and the inter-stage coupling circuit 203 connecting a coupling point H of the lower-stage filter 222b to a coupling point C of the upper-stage filter 222a. The upper-stage filter 222a comprises the input terminal 204, an upper-stage resonator 223 for selectively resonating a propagating signal included in the microwaves, the input coupling circuit 206 for coupling the input terminal 204 to a coupling point A of the resonator 223. The lower-stage filter 222b comprises a lower-stage resonator 224 for selectively resonating the propagating signal, the output terminal 208, and the output coupling circuit 209 for coupling the output terminal 208 to a coupling point F of the resonator 224. The upper-stage resonator 223 comprises the one-wavelength rectangular-shaped strip line resonator 105 and the four open-end transmission lines 64a to 64d connected to the coupling points A to D of the line resonator 105. The coupling points A,C,B and D are spaced 90 degrees in the electric length in that order, the coupling points A and D are placed at a first parallel lines L1 of the line resonator 105, and the coupling points B and C are placed at another first parallel lines L1 of the line resonator 105. A midpoint E placed in the middle of the parallel coupling line L2 of the line resonator 105 is defined, and a first electric length between the coupling point D and the midpoint E is longer than a second electric length between the coupling point B and the midpoint E. The lower-stage resonator 224 comprises the one-wavelength rectangular-shaped strip line resonator 106 and the the four open-end transmission lines 64f to 64i connected to the coupling points F to I of the line resonator 106. The coupling points I,G,H and F are spaced 90 degrees in the electric length in that order, the coupling points I and F are placed on one of first parallel lines L1 of the line resonator 106, and the coupling points G and H are placed at another of first parallel lines L1 of the line resonator 106. A midpoint K of the parallel coupling line L2 of the line resonator 106 is defined, and the first electric length between the coupling point I and the midpoint K is longer than the second electric length between the coupling point G and the midpoint K. The parallel coupling line L2 of the line resonator 105 closely faces the parallel coupling line L2 of the line resonator 106 through the parallel coupling space S6 to arrange the open-end transmission lines 64a to 64d of the line resonator 105 symmetrically to the open-end transmission lines 64f to 64i of the line resonator 106 with respect to an central line CL of the strip-line filter 221. In the above configuration, a propagating signal is resonated and filtered in the strip-line filter 221 in the same manner as in the strip-line filter 201. In this case, the depth of the notches surrounding the passband of the microwave varies by changing a difference between the first electric length and the second electric length. Also, even though an electric length of the parallel coupling lines L2 and a gap width between the upper-stage filter 222a and the lower-stage filter 222b are fixed, a coupling strength between the upper-stage filter 222a and the lower-stage filter 222b varies by changing a difference between the first electric length and the second electric length. Accordingly, the depth of the notches can be adjusted by adjusting a difference between the first electric length and the second electric length. Also, a coupling strength between the upper-stage filter 222a and the lower-stage filter 222b can be adjusted without changing an electric length of the parallel coupling lines L2 or a gap width between the upper-stage filter 222a and the lower-stage filter 222b. Therefore, the strip-line filter 221 can be maintained in a small size. Next, a second modification of the first embodiment is described with reference to Fig. 23. Fig. 23 is a plan view of a strip-line filter according to a second modification of the first embodiment. As shown in Fig. 23, a strip-line filter 231 comprises an upper-stage filter 232a, a lower-stage filter 232b coupled to the upper-stage filter 232a through the parallel coupling space S6 in electromagnetic coupling, and the inter-stage coupling circuit 203 connecting a coupling point H of the lower-stage filter 232b to a coupling point C of the upper-stage filter 232a. The upper-stage filter 232a comprises the input terminal 204, an upper-stage resonator 233 for selectively resonating a propagating signal included in the microwaves, the input coupling circuit 206 for coupling the input terminal 204 to a coupling point A of the resonator 233. The lower-stage filter 232b comprises a lower-stage resonator 234 for selectively resonating the propagating signal, the output terminal 208, and the output coupling circuit 209 for coupling the output terminal 208 to a coupling point F of the resonator 234. The upper-stage resonator 233 comprises the one-wavelength rectangular-shaped strip line resonator 105 and the four open-end transmission lines 64a to 64d connected to the coupling points A to D of the line resonator 105. The coupling points A,C,B and D are spaced 90 degrees in the electric length in that order, the coupling points A and D are placed at a first parallel lines L1 of the line resonator 105, and the coupling points B and C are placed at another first parallel lines L1 of the line resonator 105. A midpoint E placed in the middle of the parallel coupling line L2 of the line resonator 105 is defined, and a first electric length between the coupling point D and the midpoint E is longer than a second electric length between the coupling point B and the midpoint E. The lower-stage resonator 234 comprises the one-wavelength rectangular-shaped strip line resonator 106 and the the four open-end transmission lines 64f to 64i connected to the coupling points A to D of the line resonator 106. The coupling points I,G,H and F are spaced 90 degrees in the electric length in that order, the coupling points I and F are placed at a first parallel lines L1 of the line resonator 106, and the coupling points G and H are placed at another first parallel lines L1 of the line resonator 106. A midpoint K of the parallel coupling line L2 of the line resonator 106 is defined. A difference between the coupling point I and the midpoint K is set to the second electric length, and a difference between the coupling point G and the midpoint K is set to the first electric length. The parallel coupling line L2 of the line resonator 105 closely faces the parallel coupling line L2 of the line resonator 106 through the parallel coupling space S6 to arrange the open-end transmission lines 64a to 64d of the line resonator 105 symmetrically to the open-end transmission lines 64f to 64i of the line resonator 106 with respect to an central line CL of the strip-line filter 231. In the above configuration, a propagating signal is resonated and filtered in the strip-line filter 231 in the same manner as in the strip-line filter 221. Accordingly, the depth of the notches can be adjusted by adjusting a difference between the first electric length and the second electric length, in the same manner as in the strip-line filter 221. Also, a coupling strength between the upper-stage filter 232a and the lower-stage filter 232b can be adjusted without changing an electric length of the parallel coupling lines L2 or a gap width between the upper-stage filter 232a and the lower-stage filter 232b, in the same manner as in the strip-line filter 221. Therefore, the strip-line filter 231 can be maintained in a small size. Next, a third modification of the first embodiment is described with reference to Fig. 24. Fig. 24 is a plan view of a strip-line filter according to a third modification of the first embodiment. As shown in Fig. 24, a strip-line filter 241 comprises an upper-stage filter 242a, a lower-stage filter 242b coupled to the upper-stage filter 242a through the parallel coupling space S6 in electromagnetic coupling, and the inter-stage coupling circuit 203 connecting a coupling point H of the lower-stage filter 242b to a coupling point C of the upper-stage filter 242a. The upper-stage filter 242a comprises the input terminal 204, the upper-stage resonator 205, the input parallel coupling strip line 113. The lower-stage filter 242b comprises the lower-stage resonator 207, the output terminal 208, and the output parallel coupling strip line 116. In the above configuration, a propagating signal is resonated and filtered in the strip-line filter 241 in the same manner as in the strip-line filter 201. Therefore, the same effects as in the strip-line filter 201 can be obtained. Next, a fourth modification of the first embodiment is described with reference to Fig. 25. Fig. 25 is a plan view of a strip-line filter according to a fourth modification of the first embodiment. As shown in Fig. 25, a strip-line filter 251 comprises an upper-stage filter 252a, a lower-stage filter 252b coupled to the upper-stage filter 252a through the parallel coupling space S6 in electromagnetic coupling, and a pair of inter-stage paralleled coupling strip lines 253a, 253b coupled to each other for transferring a propagating signal from a coupling point H of the lower-stage filter 252b to a coupling point C of the upper-stage filter 252a. The upper-stage filter 252a comprises the input terminal 204, the upper-stage resonator 205, the input coupling circuit 206. The lower-stage filter 252b comprises the lower-stage resonator 207, the output terminal 208, and the output coupling circuit 209. In the above configuration, a propagating signal is resonated and filtered in the strip-line filter 251 through the inter-stage paralleled coupling strip lines 253a, 253b in the same manner as in the strip-line filter 201. Therefore, the same effects as in the strip-line filter 201 can be obtained. Next, a second embodiment is described with reference to Fig. 26. Fig. 26 is a plan view of a stip-line filter according to a second embodiment. As shown in Fig. 26, a strip-line filter 271 comprises an upper-stage filter 272a and a lower-stage filter 272b coupled to the upper-stage filter 272a through the parallel coupling space S6 in electromagnetic coupling. The upper-stage filter 272a comprises the input terminal 204, the upper-stage resonator 205, the input coupling circuit 206 for coupling the input terminal 204 to the coupling point A of the resonator 205, the output terminal 208, and the output coupling circuit 209 for coupling the output terminal 208 to the coupling point C of the resonator 205. The lower-stage filter 272b comprises the lower-stage resonator 207 and an internal coupling circuit 273 for transferring a propagating signal from the coupling point H to the coupling point F of the resonator 207 to change a phase of the propagating signal. In the above configuration, a propagating signal having a resonance frequency f1 is selectively resonated in the upper-stage resonator 205 and the lower-stage resonator 207 at the resonance frequency f1 according to the first resonance mode. In this case, because the coupling point D placed in the middle of the coupling points A and B is outside the parallel coupling line L2 of the line resonator 105 and because the coupling point G placed in the middle of the coupling points H and I is outside the parallel coupling line L2 of the line resonator 106, as shown in Fig. 21, a pair of notches occur in the neighborhood of a passband of microwaves including the propagating signal. Thereafter, the propagating signal is transferred from the coupling point H to the coupling point F through the internal coupling circuit 273 because the electric voltage of the coupling point H is maximized. Thereafter, the propagating signal is selectively resonated in the lower-stage resonator 207 at the resonance frequency f1 according to the second resonance mode. That is, electric voltages at the coupling points F and G reach a maximum value, and electric voltages at the coupling points H and I are zero. Thereafter, the propagating signal is transferred to the upper-stage resonator 205 through the parallel coupling space S6 and is selectively resonated at the resonance frequency f1 according to the second resonance mode. That is, electric voltages at the coupling points D and C reach a maximum value, and electric voltages at the coupling points A and B are zero. In this case, because the coupling point I placed in the middle of the coupling points F and G is outside the parallel coupling line L2 of the line resonator 106 and because the coupling point B placed in the middle of the coupling points C and D is outside the parallel coupling line L2 of the line resonator 105, the notches occurring in the neighborhood of the passband of the microwaves are deepened. Thereafter, the propagating signal is output to the output terminal 208 through the output coupling circuit 209 because the electric voltage at the coupling point C is maximized. Accordingly, the same effects as those obtained in the strip-line filter 201 can be obtained in the strip-line filter 271. An inventive idea in the second embodiment includes another inventive idea shown in the strip-line filter 201. However, as shown in Figs. 27 to 30, strip-line filters including inventive ideas shown in the strip-line filters 221, 231, 241 and 251 are also applicable. In the first and second embodiments, each of the strip-line line filters is formed of two-stage filters. However, the number of stages in the strip-line filter is not limited to two stages. That is, a multi-stage type strip-line filter can be useful.

Claims: A strip line filter (201, 221, 231, 241, 251) for resonating and filtering a microwave signal, the filter comprising:- a series of one-wavelength loop-shaped strip line resonators (105, 106) respectively having a uniform line impedance for respectively resonating and filtering a microwave signal in a first resonance mode in which electric voltages at both a first coupling point (A, I) and a second coupling point (B, H) spaced 180 degrees in electric length apart from the first coupling point are maximized and respectively resonating and filtering the microwave signal in a second resonance mode in which electric voltages at both a third coupling point (C, G) spaced 90 degrees in electric length apart from the first coupling point and a fourth point (D, F) spaced 180 degrees in electric length apart from the third coupling point are maximized, each of the resonators having a first coupling line (L 2 ) between the first and third coupling points (A, I, C, G) and a second coupling line (L 2 ) between the second and fourth coupling points (B, H, D, F); a microwave inputting element (206, 113) for inputting a microwave signal to the first coupling point (A) of the resonator arranged in the first stage; and a microwave outputting element (209, 116) for outputting the microwave signal from the fourth coupling point (F) of the resonator arranged in the final stage; the second coupling line of one resonator arranged in an N-th stage (N is an integral number) being electromagnetically coupled to the first parallel coupling line of another resonator arranged in an (N+1)-th stage to transfer the microwave signal from the resonator arranged in the N-th stage to the resonator arranged in the (N+1)-th stage; characterised by: four open-ended transmission lines (64a-d, 64f-i) connected to the first, second, third and fourth coupling points (A-D, F-I) of each of the resonators for electromagnetically influencing the microwave signal resonated in each of the resonators, the open-ended transmission lines having the same electromagnetic characteristics; and an inter-stage coupling circuit (203, 253a) for transferring the microwave signal resonating in the first resonance mode from the second coupling point (H) of the resonator (106) in the final stage to the third coupling point (C) of the resonator arranged in the first stage (105) so as to cause the microwave signal transferred by the inter-stage coupling circuit to resonate in the second resonance mode, the microwave signal resonating in the second resonance mode being output by the microwave outputting element (209, 116). A strip-line filter according to claim 1 in which said microwave inputting element is formed of a coupling strip line (113) arranged in parallel to a strip line of said resonator in the first stage, and said microwave outputting element (116) is formed of a coupling strip line arranged in parallel to a strip line of said resonator arranged in the final stage. A strip line filter (271) for resonating and filtering a microwave signal, comprising: a series of one-wavelength loop-shaped strip line resonators (105, 106) respectively having a uniform line impedance for respectively resonating and filtering a microwave signal in a first resonance mode in which electric voltages at both a first coupling point (A, I) and a second coupling point (B, H) spaced 180 degrees in electric length apart from the first coupling point are maximized and respectively resonating and filtering the microwave signal in a second resonance mode in which electric voltages at both a third coupling point (C, G) spaced 90 degrees in electric length apart from the first coupling point and a fourth coupling point (D, F) spaced 180 degrees in electric length apart from the third coupling point are maximized, each of the resonators having a first coupling line (L 2 ) between the first and third coupling points and a second coupling line (L 2 ) between the second and fourth coupling points, a microwave inputting element (206) for inputting a microwave signal to the first coupling point (A) of the resonator in a first stage; wherein the second coupling line of the resonator in an N-th stage (N is an integral number) is electromagnetically coupled to the first parallel coupling line of another resonator arranged in an (N+1)-th stage to transfer the microwave signal between the resonator arranged in the N-th stage and the resonator arranged in the (N+1)-th stage; characterised by: a microwave outputting element (209, 208) for outputting the microwave signal resonating in the second resonance mode in the resonator (105) in the first stage; four open-ended transmission lines (64a-d, 64f-i) connected to the first, second, third and fourth coupling points (A-D, F-I) of each of the resonators for electromagnetically influencing the microwave signals resonating therein, the open-ended transmission lines having the same electromagnetic characteristics; and an inter-stage coupling circuit (273) for transferring the microwave signal resonating in the first resonance mode from the second coupling point (M) of the resonator in the final stage to the fourth coupling point (F) of the resonator in the final stage, the microwave signal transferred by the inter-stage coupling circuit resonating in the second resonance mode and being transferred by stages from the resonator of the final stage to the resonator arranged in the first stage, thereby to be filtered and output by the microwave outputting element (209, 208). A strip line filter according to any one of claims 1 to 3, in which the first and second parallel coupling lines (L 2 ) are respectively shorter than 90 degrees in electric length. A strip line filter according to any one of claims 1 to 4, in which the resonators are respectively in a rectangular shape, the resonators respectively have two first straight lines longer than 90 degrees in electric length and two second straight lines shorter than 90 degrees in electric length, the first and fourth coupling points (A, D; F, I) are placed at the same first parallel line of each of the one-wavelength loop-shaped strip line resonators, the second and third coupling points (B, C; G, M) are placed at the other first parallel line of each of the resonators, and the first and second coupling lines are formed of the second straight lines of each of the resonators. A strip line filter according to claim 5, in which a first electric length between a first midpoint (K) placed in the middle of the first coupling line and the first coupling point is equal to a second electric length between the first midpoint (K) and the third coupling point, a third electric length between a second midpoint (E) placed in the middle of the second coupling line and the second coupling point is equal to a fourth electric length between the second midpoint (E) and the fourth coupling point, and the first electric length is equal to the third electric length. A strip line filter according to claim 5 in which a first electric length between a first midpoint (K) placed in the middle of the first coupling line and the first coupling point is longer than a second electric length between the first midpoint (K) and the third coupling point, a third electric length between a second midpoint (E) placed in the middle of the second coupling line and the second coupling point is shorter than a fourth electric length between the second midpoint (E) and the fourth coupling point, the first electric length is equal to the fourth electric length, and the second electric length is equal to the third electric length. A strip line filter according to claim 5, in which a first electric length between a first midpoint (K) placed in the middle of the first coupling line and the first coupling point is shorter than a second electric length between the first midpoint (K) and the third coupling point, a third electric length between a second midpoint (E) placed in the middle of the second coupling line and the second coupling point is shorter than a fourth electric length between the second midpoint (E) and the fourth coupling point, the first electric length is equal to the third electric length, and the second electric length is equal to the fourth electric length. A strip line filter according to claim 3, or any claim dependent thereon in which the microwave inputting element is formed of a coupling strip line (113) arranged in parallel to a strip line of the resonator (105) arranged in the first stage, and the microwave outputting element is formed of a coupling strip line (16) arranged in parallel to a strip line of the resonator (105) arranged in the first stage. A strip-line filter according to any one of claims 1 to 9, in which the inter-stage coupling circuit is formed of a pair of parallel strip lines (253a, 253b) coupling to each other.

Inventors: MAKIMOTO MITSUO, MATSUO MICHIAKI, SAGAWA MORIKAZU, YABUKI HIROYUKI, MAKIMOTO, MITSUO, MATSUO, MICHIAKI, SAGAWA, MORIKAZU, YABUKI, HIROYUKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1994
Main classification: H01P1
IPC codes: H01P7, H01P1